Nanog is a homeoprotein that functions with pluripotent factors such as Oct4 and Sox2 to maintain embryonic stem cell pluripotency. Expression of this protein has been noted in seminoma, dysgerminoma, embryonal carcinoma, and other undifferentiated germ cell tumors, while nanog expression is absent in normal adult organ tissues. Anti-Nanog may be useful in distinguishing between undifferentiated germ cell tumors and non-germ cell tumors.